The present invention relates to an electrorheological (ER) fluid composition. More particularly, this invention relates to an ER fluid comprising a solid phase dispersed in a base liquid wherein the base liquid comprises a miscible mixture of an alkylmethylsiloxane fluid and one or more organofluoro compounds such that the miscible base fluid mixture has a specific gravity within 0.2 of the solid phase.
When certain polarizable solid particles are dispersed in an electrically non-conducting hydrophobic liquid, the resulting suspensions exhibit peculiar rheological properties under the influence of an electrical field. These systems show a dramatic increase in viscosity and modulus with applied voltage, in some cases literally being transformed from a liquid to a virtual solid upon the application of the electric field. This change is reversible and typically takes place in a matter of milliseconds. Materials which exhibit this phenomenon are called electrorheological (ER) or electroviscous (EV) fluids, and have been known for at least the last fifty years. These fluids find utility in such areas as torque transfer and mechanical damping applications.
The early ER fluids comprised such systems as starch dispersed in transformer oil or silica gel dispersed in kerosine or mineral oil. Since these early discoveries, only a relatively small number of improvements over old ones have emerged in this art.
It is desirable in the ER fluid art to improve the strength of such fluids which thereby permits smaller devices requiring less power to be utilized. The production of an ER fluid with greater strength would also allow devices to be operated at lower voltages, which would have advantages in power supply design, and generally would open up other application areas for the use of ER fluids that are currently beyond the capabilities of existing ER fluids.
Polychlorotrifluoroethylene (CTFE) when blended with hydrophilic particles functions as an active base fluid for electrorheological (ER) fluid systems. However its high specific gravity (1.9) often limits its utility with lower specific gravity particle systems based on organic or siloxane polymers due to the poor stability of the resulting dispersion. Also, the loss currents of many ER fluids based on CTFE have been found to be prohibitively high.
Fluids which are effective base fluids have been described in the ER fluid art. For example, Stangroom, in Great Britain patent specification No. 1,570,234 (to the Secretary of State of Defense, London) teaches an ER fluid comprising a lithium salt of polymethacrylic acid (LMAA) as the solid phase, and a chlorinated paraffin as the base liquid. Although these ER fluids have been moderately successful, they are nonetheless deficient in a number of properties. For example, their zero-field viscosity is relatively high, which in some instances can make it difficult to control the fluid. Also, they have a relatively high pour-point, resulting in an undesirably high viscosity at low temperatures, while on the other hand at high temperatures they start to decompose to highly corrosive by- products including hydrochloric acid. Therefore the useful temperature range of these ER fluids is limited which prevents their widespread adoption in many industries eg. the automotive industry, where ER fluids could otherwise be useful.
ER fluids employing silicone oil as the base fluid phase have also been disclosed. For example, Goossens et al., in U.S. Pat. No. 4,645,614, teaches an electroviscous suspension which is based on a mixture of aqueous silica gel with silicone oil as the liquid phase to which a dispersant is added. The dispersant consists of amino, hydroxy, acetoxy, or alkoxy functional polysiloxanes having a molecular weight above 800. The electroviscous suspensions are disclosed as being highly compatible with elastomeric materials, non-sedimenting, non-flammable and physiologically acceptable. They are also described as heat and freeze resistant over a wide temperature range and are largely unaffected by temperature and pressure in their viscosity.
Carlson, in U.S. Pat. No. 5,032,307 teaches an ER material containing a carrier fluid, an anionic surfactant particle component, and an activator. The non-abrasive anionic surfactant acts as both a particle component and a surfactant and the ER material is miscible with water and will not mar the surface of objects utilized in an ER device. The preferred carrier fluids of Carlson are silicone oils having viscosities of between about 0.65 and 1000 milliPascal seconds (mPa.s).
Stangroom, in U.S. Pat. No. 4,812,251, teaches an ER fluid comprising a hydrophilic solid and a hydrophobic liquid component wherein the hydrophobic liquid component comprises a fluorosilicone whose average molecular weight is in the range of 200-700. The reduction of the molecular weight of the fluorosilicone of Stangroom to the above described range is disclosed as having two desirable effects, one is that it reduces the viscosity of the fluorosilicone itself, and secondly it renders the fluorosilicone miscible with CTFE. However addition of the fluorosilicone fluids has done little to reduce the loss currents of such systems.
Siloxanes have also been disclosed in the ER fluid art as being useful as base fluids. For example, Brooks et al. in Great Britain Unexamined Application No. 2210893, teaches an ER fluid comprising a solid phase dispersed in a base liquid which is characterized in that the base liquid comprises a polyfluoroalkylmethylsiloxane. The ER fluids of Brooks et al. are disclosed as having improved strength and stability and are taught as being useful in fluid power systems and engineering applications such as in clutches, brake systems, fluid drives, and couplings.
Hashimoto et al., in Japanese Patent Application Laid Open No. 01304144, teaches an electroviscous liquid which comprises an inorganic solid or fine powder dispersion modified with an alkoxysilane. The liquid is prepared by dispersing an inorganic solid or inorganic fine powder in water or organic solvent, and then modifying the resulting dispersion with an alkoxysilane having hydrophobic substitution, the substitutes being monovalent and divalent aliphatic, aromatic or unsaturated hydrocarbons. An emulsion results which is then added to silicone oil to prepare the final product of electroviscous liquid. Preferred silicone oils to be used as dispersion media for the electroviscous liquid of Hashimoto et al. include homopolymers or copolymers made of units selected from among polydimethylsiloxane, polymethylphenylsiloxane, polydiphenylsiloxane, polymethylchlorophenylsiloxane, polymethyl-long-chain-alkylsiloxane, polymethylcyanopropylsiloxane, and polymethyl-3,3,3-trifluoromethylsiloxane as well as their mixtures.
However, none of the references described hereinabove teach a mixture of linear and/or cyclic alkylmethylsiloxane oils and organofluoro compounds as base fluids which provide improved ER performance properties and much improved lubricity in comparison to polydimethylsiloxane based ER fluids. The present invention also teaches how to obtain miscible base fluids enabling control of the specific gravity of the base fluid mixture, the range of which is controlled by the specific gravity and concentration of the mixture components such that it can be matched with that of the dispersed phase to provide enhanced dispersion stability.